Gate valve with pressure actuated plug seat

ABSTRACT

A gate valve including a body having inlet and outlet passages intersected by a cylindrical bore receiving a pair of spaced, opposed seal plates having passages aligned with the other passages, the exterior of the seal plates being cylindrical, conforming to the wall of the cylindrical bore and sealing thereagainst, the inner surfaces of the seal plates being flat and parallel to each other. A gate or carrier located between and slidable along the flat faces has a passage alignable with all of the other passages to produce full opening of the valve, the gate carrying a cylindrical plug seat in a companion cylindrical gate cavity which is slidable with the gate along the flat surfaces across the seal plate passages to close the gate valve, the upstream pressure shifting the plug seat in the gate against the flat surface of the downstream seal plate to provide a metal-tometal seal thereagainst, supplemented by a seal ring on the plug seat sealing against the gate and the downstream seal plate.

States Patent 1 Oct. 30, 1973 GATE VALVE WITH PRESSURE ACTUATED PLUGSEAT [75] Inventor: Benton F. Baugh, Houston, Tex. [73] Assignee: VetcoOffshore Industries, 1nc.,

Ventura, Calif.

[22] Filed: June 16, 1971 [21] Appl. No.: 153,733

[52] US. Cl. 251/175, 251/196, 251/327, 251/329 [51] Int. Cl F16k 3/18[58] Field of Search 251/175, 193, 196, 251/327, 328, 329, 195;137/4542, 454.6

[56] References Cited UNITED STATES PATENTS 3,194,259 7/1965 Garrod137/4542 1,871,965 8/1932 DuBois 251/175 3,580,542 5/1971 Wright 251/1753,575,377 4/1971 Carlton... 251/175 3,306,321 2/1967 Bastle 251/175FOREIGN PATENTS 0R APPLICATIONS 234,073 4/1969 U.S.S.R 251/175 8/ 86 J75 88 W W"; Q0

7a T 66 66-: f 34 L a? w i 5 38 a5 new 2%. Y I90.

Primary Examiner-Harold W. Weakley AttorneyBemard Kriegel et al.

[5 7 1 ABSTRACT A gate valve including a body having inlet and outletpassages intersected by a cylindrical bore receiving a pair of spaced,opposed seal plates having passages aligned with the other passages, theexterior of the seal plates being cylindrical, conforming to the wall ofthe cylindrical bore and sealing thereagainst, the inner surfaces of theseal plates being flat and parallel to each other. A gate or carrierlocated between and slidable along the flat faces has a passagealignable with all of the other passages to produce full opening of thevalve, the gate carrying a cylindrical plug seat in a companioncylindrical gate cavity which is slidable with the gate along the flatsurfaces across the seal plate passages to close the gate valve, theupstream pressure shifting the plug seat in the gate against the flatsurface of the downstream seal plate to provide a metal-to-metal sealthereagainst, supplemented by a seal ring on the plug seat sealingagainst the gate and the downstream seal plate.

12 Claims, 6 Drawing Figures Patented Oct. 30, 1973 ,3 Sheets-Sheet 1INVENTOR BEN TONI: .B ,qusl

jammy Patented Oct. 30, 1973 3,768,774

3 SheetS-Sheet 5 INENTOE- ENTO/V}:

f leyj GATE VALVE WITH PRESSURE ACTUATED PLUG SEAT The present inventionrelates to valve devices, and more particularly to gate valves.

Prior gate valves have been comparatively costly to manufacture,requiring special machinery to produce the gate and cavity in the valvebody, such as broaching or anocutting equipment. Moreover, the gate,which is shiftable in the valve body between valve opening and closingpositions, tend to cock slightly, preventing its floating movement bythe upstream fluid pressure fully against the downstream seal surface,thereby forestalling the securing of a desirable metal-to-metal seal.The cocking tendency may be overcome by undertaking specificmanipulative steps in operating the valve, or by providing preciselypositioned stops limiting movement of the gate in both valve opening andvalve closing directions. The prior gate valves have the furtherdisadvantage of embodying expendable parts which are relatively costlyto manufacture.

By virtue of the present invention, gate valves are provided which arematerially less costly to manufacture, requiring no specializedmachinery or equipment. Expendable parts are not threaded, as in priorvalve devices, and are, therefore, less costly to produce. An effectivemetal-to-metal seal is secured between a gate structure and a seal platewithout the need for special valve manipulations and precise locationsof limit stops. A plug seat, forming part of the gate structure, isfloatingly carried by the gate and can move relative thereto into flat,metal-to-metal sealing engagement with a companion member, themetal-to-metal seal being backed up or supplemented by a resilient seal,thereby insuring against valve leakage.

This invention possesses many other advantages, and has other purposeswhich may be made more clearly apparent from a consideration of a formin which it may be embodied. This form is shown in the drawingsaccompanying and forming part of the present specification. It will nowbe described in detail, for the purpose of illustrating the generalprinciples of the invention; but it is to be understood that suchdetailed description is not to be taken in a limiting sense.

Referring to the drawings:

FIG. 1 is a vertical section through a gate valve embodying theinvention;

FIG. 2 is an enlarged cross-section taken along the line 2-2 on FIG. 1;

FIG. 3 is a vertical section taken along the line 33 on FIG. 1, partsbeing disclosed in elevation;

FIG. 4 is a view similar to FIG. 1 illustrating the gate valve in aclosed position;

FIG. 5 is an enlarged section taken along the line 55 on FIG. 4; and

FIG. 6 is an exploded view of portions of the gate valve.

The gate valve illustrated in the drawings includes a main valve body 10having aligned inlet and outlet passages 11, 12, fluid from an inletpipe 13 being threadedly connected to the body and communicating withthe inlet passage, while fluid in the outlet passage 12 flows into anoutlet pipe 14 threadedly connected to the body. The body has acylindrical bore 15 formed therein, as by drilling, which is normal tothe inlet and outlet passages 11, 12, this bore extending from the upperend 16 of the body 10 and being closed at its lower end 17 by the valvebody itself. Disposed within the cylindrical bore are a pair of opposedseal plates l8, 19 having external or outer cylindrical surfaces 20conforming to and engaging the wall 21 of the cylindrical bore 15, andflat parallel inner surfaces 22, 23 separated from one another toprovide a space 24 receiving a gate structure 25. The lower ends 26 ofthe seal plates 18, 19 bear against the bottom of the cylindrical bore15, their upper ends 18a, 19a projecting a short distance above theupper end 16 of the main valve body 10. The seal plates have passages27, 28 of substantially the same diameters as, and aligned with, theinlet and outlet body passages 11, 12, leakage of fluid along thecylindrical exteriors of each seal plate being prevented by an elastomerseal ring 29 in a groove 30 in the cylindrical surface of each sealplate, circumscribing the seal plate passage 27 or 28, and which engagesthe companion cylindrical wall 21 of the bore 15 in surrounding relationto an adjacent body passage 11 or 12. The cylindrical surface 20 of aseal plate and the seal ring 29 surrounding its passage are disclosedmost clearly in FIG. 6.

The seal plates 18, 19 are secured in a fixed position in the main valvebody 10 by a body bonnet 31 having a bore 32 receiving the upper ends18a, 19a of the seal plates, the wall of the bore having substantiallythe same diameter as the cylindrical surfaces 20 of the seal plates. Thebonnet is firmly secured to the main valve body by a plurality of nuts33 threaded on studs 34 extending through the lower bonnet flange 35 andsecured to the main valve body 10, a suitable gasket or seal ring 36 inthe bonnet engaging the upper end 16 of the valve body to preventleakage therebetween. The seal plates 18, 19 are properly orientedwithin the valve body 10 to insure proper alignment of their passages27, 28 with the inlet and outlet passages 11, 12 by a pin 37 secured tothe bonnet and being placed selectively in one of a plurality ofcircumferentially disposed sockets 38 in the upper end portions of theseal plates, the bonnet 31 itself being prevented from turning relativeto the main valve body 10 by circumferentially spaced pins 39 secured insockets 40 in the upper portion of the valve body and extending intoopposed companion sockets 41 in the bonnet.

The gate structure 25 includes a gate 42 having opposed flat parallelsides 43, 43 spaced from one another a distance conforming to thespacing between the inner flat sides 22, 23 on the seal plates 18, 19and is shiftable from a valve opening position in which a gate passage44, conforming to the other passages 27, 28, in aligned therewith, inwhich the valve is fully open, to a position in which a portion of thegate structure obstructs the passages 27, 28 to close the valve. Thus,the gate structure includes a generally cylindrical plug seat 50 mountedwithin a cylindrical bore 45 extending through the gate 42, the axis ofthe bore and of the plug seat being parallel to the passages 11, 12, 27,28 throughthe main valve body 10 and seal plates 18, 19. The overallthickness or axial extent of the plug seat 44 is the same as thethickness of the gate 42, the gate itself having oppositely directedcylindrical convex surfaces 46 conforming to the wall 21 of thecylindrical body bore 15 and slidable therealong. The flat ends 47 ofthe plug seat 50 are spaced from one another a distance substantiallyequal to the distance between the inner flat sides 22, 23 of the sealplates 18, 19, the diameter of the plug seat being substantially greaterthan the diameter of the aligned passages 27, 28 through the seal platesso that the plug seat can fully bridge the seal plate passages with theouter marginal portions of the plug seat engaging the flat sides 22, 23of the seal plates 18, 19, for the purpose of forming a metal-to-metalseal thereagainst.

As assurance against leakage between the plug seat 50 and the gate 42,the plug seat carries circular elastomer seal rings 51 in its peripherythat sealingly engage against the wall 52 of the cylindrical bore 45 ofthe gate. As disclosed, end seal rings 51 are mounted in peripheralgrooves 53 in the plug seat, the seal rings having skirt portions 54projecting in opposite directions from each other which engage thecylindrical wall 52 of the gate bore, each skirt terminating at theadjacent flat surface 47 of the plug seat and being adapted to contactthe flat side 22 or 23 of an adjacent seal plate. Thus, the seal rings51 prevent leakage of fiuid between the plug seat fill and thecylindrical wall 52 of the gate in both directions, and they also sealagainst the flat surfaces 22, 23 of the seal plates 18, 19.

A rising valve stem 60 is secured to the gate 42 for the purpose ofshifting the latter transversely of the passages 27, 28, thereby movingthe gate and its plug seat 50 between valve opening and valve closingpositions. The stem section 60 is coupled to the gate by disposing thestem within a bore 61 in the gate that opens through its upper end andinserting pins 62 on opposite sides of the stem transversely across thestem, each pin being received within a cylindrical recess 63 in the stemand an opposed cylindrical recess 64 in the gate 42. Thus, in connectingthe stem section 60 to the gate, the stem need only be moved into thebore 61 of the gate, with its cylindrical recesses 63 confronting thecylindrical recesses 64 in the gate, whereupon the connector pins 62 areinserted into the confronting recesses. The pins 62 have a lengthcorresponding to the thickness of the gate 42 and cannot becomedislodged when the gate is inserted between the seal plates 18, 19,since they are retained in position by the flat sides 22, 23 of the sealplates.

The stem 60, gate 42 and plug seat 50 are shiftable in a transversedirection with respect to the valve body and seal plate passages 11, 12,27, 28 in any suitable manner. As disclosed, the shifting action isperformed mechanically. The rising stem section 60 is externallythreaded and is threadedly received within an internally threaded bore66 in a rotatable stem section 67 disposed within an upper portion orskirt 68 of the bonnet, a cylindrical reduced diameter rotatable stemportion 69 extending through a packing gland 70 threaded into the upperend of the bonnet skirt and terminating thereabove. The inner end of thepacking gland abuts a stem packing 71 surrounding and sealingly engagingthe upper stem portion 69, as well as the inner wall '72 of the bonnet,to prevent fluid leakage along the stem. A suitable lubricant can beforced into the bonnet through a grease fitting 73 threaded thereinto,this lubricant being capable of passing through ports 74 in therotatable stem section into the threaded bore 66.

Axial movement of the rotatable stem section 67 is prevented by a thrustbearing assembly 75 mounted at the upper end of the bonnet. Thus, anupper stern section 76 has an inner bore 77 in which the upper end ofthe cylindrical stem portion 6Q is inserted, the stem sections 76, 67being secured together by an overload pin 73 extending transverselytherethrough. The upper stem section includes a flange 79, the innersurface of which engages an inner thrust bearing 80, contacting theupper end of the packing gland 70, the outer surface of the flangeengaging an outer thrust bearing 81 that engages an inwardly directedflange 82 of a cap 83 threadedly secured on the upper end of the bonnetskirt 68. Leakage of fluid between the cap and the upper stem section isprevented by a suitable seal ring 84 mounted in the flange 82 of the capand sealing against the upper stem section 76. It is, therefore,apparent that rotation of the upper stem section 76 in either directionwill be transmitted through the overload pin 78 to the inner stemsection 67. The required rotary effort can be secured from a suitablewrench (not shown) placed against the flats 85 provided on the upperstem section 76.

The bearings 80, 81 can be lubricated through grease fittings 86 mountedin the cap 83 and communicating with passages 87 extending to the upperand lower bearings. Excess lubricant can pass from the end cap through asuitable bent passage 88 communicating with a peripheral groove 89 inthe exterior of the cap, a suitable elastomer ring 90 being disposed inthis groove which will permit outward passage of lubricant from the ventpassage 88, but which will engage the base of the peripheral groove 89to preclude reverse flow of fluid through the passage.

As disclosed in FIG. 1, the gate valve is in an open position, the gatepassage 44 being aligned with the seal plate and body passages 27, 28,11, 12. The threaded connection between the rotatable and rising stemsections 67, 60 may be of any suitable hand. As disclosed, a left-handthreaded arrangement is provided. Accordingly, application of a wrenchto the wrench flats 85 and rotation of the stem sections 76, 67 to theleft will cause the inner stem section 60 to move upwardly within thebonnet 31 and shift the gate valve structure 25 upwardly along the sealplates 18, 19, until the plug seat 50 is disposed fully across the sealplate passages 27, 28, as disclosed in FIGS. 4 and 5. The plug seat 50can be forced by the upstream fluid pressure in a downstream directionwithin the cylindrical bore 45 of the gate into metal-to-metal sealingcontact with the flat surface 23 of the downstream seal plate 19, fullyclosing its seal passage 28. This metal-tometal seal is supplemented bythe engagement of the downstream elastomer seal 51 with the flat surface23 of the downstream seal plate. The upstream elastomer seal 51 will beretained by the upstream fluid pressure in leakproof sealing engagementwith the wall 52 of the cylindrical bore 45 through the gate 42. Fluidpressure cannot leak between the seal plates 18, 19 and the cylindricalwall 21 of the valve body bore 15 because of the sealing engagement ofthe sealrings 29 on the seal plates therewith.

Rotation of the upper stem section 76 and rotatable stem section 67 inthe opposite or right direction will shift the gate structure 25,including its movable plug seat 50, in the opposite direction, returningsuch structure to the full valve opening position disclosed in FIG. 1.

Because of the gate valve arrangement illustrated and described, it canbe manufactured at less cost than prior gate valves. The gate cavity 15is a round or cylindrical bored hole. It does not require casting,breaching or anocutting with the special machinery and equipmentrequired to perform such operations. Cocking of the gate 42 will notaffect the full metal-to-metal seal between the plug seat 50 and thedownstream face 23 of the seal plate, since the plug seat is floating inthe gate 42 and can shift relative to the gate into proper engagementwith the seal plate 19. Special stops and manipulations are unnecessaryin securing the proper and full closing of the gate valve, as well asits full opening. Although the metal-to-metal seal between the plug seat50 and the seal plate 19 will prevent leakage, the elastomer orresilient seal 51 is still available for preventing leakage,supplementing the metal-to-metal seal. In the event that the valvebecomes defective in its operation, the plug seat 50 is a relativelyinexpensive part to replace. For that matter, if the gate 42 requiresreplacement, its cost is less than prior gates, since it does not embodya threaded portion and is, therefore, more economical to produce.

I claim:

1. In a valve: a body having an inlet passage, an outlet passage, and asubstantially cylindrical bore extending across said passages; opposedindividual separate segment seal plates removably disposed in said boreand having external cylindrical surfaces conforming to and engaging thecylindrical wall of said bore, said plates having confronting flat innerupstream and downstream surfaces spaced from and parallel to oneanother, said plates having inlet and outlet passages, respectively;means for retaining said plates in their respective positions in saidbore; a gate structure including a gate between said seal plates andhaving opposed flat surfaces slidable along said confronting flat innersurfaces, said gate having a flow passage communicable with said platepassages when said structure is in a first position between said plates,said gate further having a cylindrical gate bore paralleling said flowpassage; a generally cylindrical one piece plug seat slidably mounted insaid gate bore and having opposed flat parallel upstream and downstreamsurfaces, said plug seat being subject to fluid pressure in said inletpassages acting upon said upstream seat surface to force said downstreamseat surface into sealing engagement against said downstream platesurface when said surface occupies a second position between said plateswith said plug seat extending across said plate inlet and outletpassages to prevent fluid flow therebetween; and means for shifting saidgate structure between said first and second positions.

2. In a valve as defined in claim 1; said gate having cylindricalsurfaces conforming to and slidable along the cylindrical wall of saidbody bore between said segment seal plates.

3. In a valve as defined in claim 1; and seal means on said plug seatsurrounding said plate outlet and engageable with said downstream flatsurface to supplement the sealing engagement of said plug seat surfaceagainst said downstream flat surface when said structure occupies saidsecond position.

4. In a valve as defined in claim 1; said gate having cylindricalsurfaces conforming to and slidable along the cylindrical wall of saidbody bore between said segment seal plates; and seal means on said plugseat surrounding said plate outlet and engageable with said downstreamflat surface to supplement the sealing engagement of said plug seatsurface against said downstream flat surface when said structureoccupies said second position.

5. In a valve as defined in claim 1; and a seal ring on said plug seatsealingly engaging the wall of said cylindrical gate bore andsurrounding said plate outlet and engageable with said downstream flatsurface surface to supplement the sealing engagement of said plug seatsurface against said downstream flat surface when said structureoccupies said second position.

6. In a valve: a body having an inlet passage, an outlet passage, and asubstantially cylindrical bore extending across said passages; opposedindividual segment seal plates removably disposed in said bore andhaving external cylindrical surfaces conforming to and engaging thecylindrical wall of said bore, said plates having con fronting flatinner upstream and downstream surfaces spaced from and parallel to oneanother, said plates having inlet and outlet passages communicating withsaid body inlet and outlet passages, respectively; a gate structureincluding a gate between said seal plates and having opposed flatsurfaces slidable along said confronting flat inner surfaces, said gatehaving a flow passage communicable with said plate passages when saidstructure is in a first position between said plates, said gate furtherhaving a cylindrical gate bore paralleling said flow passage; agenerally cylindrical plug seat slidably mounted in said gate bore andhaving opposed flat parallel upstream and downstream surfaces, said plugseat being subject to fluid pressure in said inlet passages acting uponsaid upstream seat surface to force said downstream seat surface intosealing engagement against said downstream plate surface when saidstructure occupies a second position between said plates with said plugseat extending across said plate inlet and outlet passages to preventfluid flow therebetween; and means for shifting said gate structurebetween said first and second positions; and seal means on said plugseat surrounding said plate outlet and engageable with said downstreamflat surface to supplement the sealing engagement of said plug seatsurface against said downstream flat surface when said structureoccupies said second position; and a seal ring on said plug seatadjacent said upstream seat surface and engageable with the wall of saidcylindrical gate bore to prevent leakage of fluid from said inletpassages around said plug seat.

'7. In a valve as defined in claim 1; and a seal ring on said plug seatsealingly engaging the wall of said cylindrical gate bore andsurrounding said plate outlet and engageable with said downstream flatsurface to supplement the sealing engagement of said plug seat surfaceagainst said downstream flat surface when said structure occupies saidsecond position; said gate having cylindrical surfaces conforming to andslidable along the cylindrical wall of said body bore between saidsegment seal plates.

8. In a valve as defined in claim 1; and seal means on the exteriors ofsaid seal plates surrounding said inlet passages and outlet passages andsealingly engaging the cylindrical wall of said body bore.

9. In a valve as defined in claim 1; said gate having cylindricalsurfaces conforming to and slidable along the cylindrical wall of saidbody bore between said segment seal plates; and seal means on theexteriors of said seal plates surrounding said inlet passages and outletpassages and sealingly engaging the cylindrical wall of said body bore.

10. In a valve: a body having an inlet passage, an outlet passage, and asubstantially cylindrical bore extending across said passages; opposedindividual segment seal plates removably disposed in said bore andhaving external cylindrical surfaces conforming to and engaging thecylindrical wall of said bore, said plates having confronting flat innerupstream and downstream surfaces spaced from and parallel to oneanother, said plates having inlet and outlet passages communicating withsaid body inlet and outlet passages, respectively; a gate structureincluding a gate between said seal plates and having opposed flatsurfaces slidable along said confronting flat inner surfaces, said gatehaving a flow passage communicable with said plate passages when saidstructure is in a first position between said plates, said gate furtherhaving a cylindrical gate bore paralleling said flow passage; agenerally cylindrical plug seat slidably mounted in said gate bore andhaving opposed flat parallel upstream and downstream surfaces, said plugseat being subject to fluid pressure in said inlet passages acting uponsaid upstream seat surface to force said downstream seat surface intosealing engagement against said downstream plate surface when saidstructure occupies a second position between said plates with said plugseat extending across .said plate inlet and outlet passages to preventfluid flow therebetween; and means for shifting said gate structurebetween said first and second positions; and a seal ring on said plugseat sealingly engaging the wall of said cylindrical gate bore andsurrounding said plate outlet and engageable with said downstream flatsurface to supplement the sealing engagement of said plug seat surfaceagainst said downstream flat surface when said structure occupies saidsecond position; and a seal ring on said plug seat adjacent saidupstream seat surface and engageable with the wall of said cylindricalgate bore to prevent leakage of fluid from said inlet passages aroundsaid plug seat.

11. In a valve as defined in claim 10; said gate having cylindricalsurfaces conforming to and slidable along the cylindrical wall of saidbody bore between said segment seal plates.

12. In a valve as defined in claim 10; and seal means on the exteriorsof said seal plates surrounding said inlet passages and outlet passagesand sealingly engaging the cylindrical wall of said body bore.

7 7 3 UIQITED STATES PATENT 013 161") I CER'QH lCAl 1. Ci LOKEULC l LQNPatent; NO. Dated 30,

BET0N F. BAUGH lnvcntor(s) is certified that error appears in theabove-identified patent and t: 1: said Letters Patent are hctcbycorrected as shown below:

Column '1, line 11': "tend should be "tendsn Col 4, line 21: "bent"should be "vent- Column 5,-lihe 27; after 'f havi ng" inssr t "inlet and'outlet I paissagescommunicating with said body".

line 43: cancel "surface" (second occurrence and substitute structute-Column 6, line A n: cancel "surface" (last occurrsnce).

Signed and sealed this 23rd day of" April 197M...

I QSQAL} fittest:

- c. i'iAfl Pix L DAMN AEESS tlTLE QfflC'SZ I Commissioner or Patent:

1. In a valve: a body having an inlet passage, an outlet passage, and asubstantially cylindrical bore extending across said passages; opposedindividual separate segment seal plates removably disposed in said boreand having external cylindrical surfaces conforming to and engaging thecylindrical wall of said bore, said plates having confronting flat innerupstream and downstream surfaces spaced from and parallel to oneanother, said plates having inlet and outlet passages, respectively;means for retaining said plates in their respective positions in saidbore; a gate structure including a gate between said seal plates andhaving opposed flat surfaces slidable along said confronting flat innersurfaces, said gate having a flow passage communicable with said platepassages when said structure is in a first position between said plates,said gate further having a cylindrical gate bore paralleling said flowpassage; a generally cylindrical one piece plug seat slidably mounted insaid gate bore and having opposed flat parallel upstream and downstreamsurfaces, said plug seat being subject to fluid pressure in said inletpassages acting upon said upstream seat surface to force said downstreamseat surface into sealing engagement against said downstream platesurface when said surface occupies a second position between said plateswith said plug seat extending across said plate inlet and outletpassages to prevent fluid flow therebetween; and means for shifting saidgate structure between said first and second positions.
 2. In a valve asdefined in claim 1; said gate having cylindrical surfaces conforming toand slidable along the cylindrical wall of said body bore between saidsegment seal plates.
 3. In a valve as defined in claim 1; and seal meanson said plug seat surrounding said plate outlet and engageable with saiddownstream flat surface to supplement the sealing engagement of saidplug seat surface against said downstream flat surface when saidstructure occupies said second position.
 4. In a valve as defined inclaim 1; said gate having cylindrical surfaces conforming to andslidable along the cylindrical wall of said body bore between saidsegment seal plates; and seal means on said plug seat surrounding saidplate outlet and engageable with said downstream flat surface tosupplement the sealing engagement of said plug seat surface against saiddownstream flat surface when said structure occupies said secondposition.
 5. In a valve as defined in claim 1; and a seal ring on saidplug seat sealingly engaging the wall of said cylindrical gate bore andsurrounding said plate outlet and engageable with said downstream flatsurface to supplement the sealing engagement of said plug seat surfaceagainst said downstream flat surface when said structure occupies saidsecond position.
 6. In a valve: a body having an inlet passage, anoutlet passage, and a substantially cylindrical bore extending acrosssaid passages; opposed individual segment seal plates removably disposedin said bore and having external cylindrical surfaces conforming to andengaging the cylindrical wall of said bore, said plates havingconfronting flat inner upstream and downstream surfaces spaced from andparallel to one another, said plates having inlet and outlet passagescommunicating with said body inlet and outlet passages, respectively; agate structure including a gate between said seal plates and havingopposed flat surfaces slidable along said confronting flat innersurfaces, said gate having a flow passage communicable with said Platepassages when said structure is in a first position between said plates,said gate further having a cylindrical gate bore paralleling said flowpassage; a generally cylindrical plug seat slidably mounted in said gatebore and having opposed flat parallel upstream and downstream surfaces,said plug seat being subject to fluid pressure in said inlet passagesacting upon said upstream seat surface to force said downstream seatsurface into sealing engagement against said downstream plate surfacewhen said structure occupies a second position between said plates withsaid plug seat extending across said plate inlet and outlet passages toprevent fluid flow therebetween; and means for shifting said gatestructure between said first and second positions; and seal means onsaid plug seat surrounding said plate outlet and engageable with saiddownstream flat surface to supplement the sealing engagement of saidplug seat surface against said downstream flat surface when saidstructure occupies said second position; and a seal ring on said plugseat adjacent said upstream seat surface and engageable with the wall ofsaid cylindrical gate bore to prevent leakage of fluid from said inletpassages around said plug seat.
 7. In a valve as defined in claim 1; anda seal ring on said plug seat sealingly engaging the wall of saidcylindrical gate bore and surrounding said plate outlet and engageablewith said downstream flat surface to supplement the sealing engagementof said plug seat surface against said downstream flat surface when saidstructure occupies said second position; said gate having cylindricalsurfaces conforming to and slidable along the cylindrical wall of saidbody bore between said segment seal plates.
 8. In a valve as defined inclaim 1; and seal means on the exteriors of said seal plates surroundingsaid inlet passages and outlet passages and sealingly engaging thecylindrical wall of said body bore.
 9. In a valve as defined in claim 1;said gate having cylindrical surfaces conforming to and slidable alongthe cylindrical wall of said body bore between said segment seal plates;and seal means on the exteriors of said seal plates surrounding saidinlet passages and outlet passages and sealingly engaging thecylindrical wall of said body bore.
 10. In a valve: a body having aninlet passage, an outlet passage, and a substantially cylindrical boreextending across said passages; opposed individual segment seal platesremovably disposed in said bore and having external cylindrical surfacesconforming to and engaging the cylindrical wall of said bore, saidplates having confronting flat inner upstream and downstream surfacesspaced from and parallel to one another, said plates having inlet andoutlet passages communicating with said body inlet and outlet passages,respectively; a gate structure including a gate between said seal platesand having opposed flat surfaces slidable along said confronting flatinner surfaces, said gate having a flow passage communicable with saidplate passages when said structure is in a first position between saidplates, said gate further having a cylindrical gate bore parallelingsaid flow passage; a generally cylindrical plug seat slidably mounted insaid gate bore and having opposed flat parallel upstream and downstreamsurfaces, said plug seat being subject to fluid pressure in said inletpassages acting upon said upstream seat surface to force said downstreamseat surface into sealing engagement against said downstream platesurface when said structure occupies a second position between saidplates with said plug seat extending across said plate inlet and outletpassages to prevent fluid flow therebetween; and means for shifting saidgate structure between said first and second positions; and a seal ringon said plug seat sealingly engaging the wall of said cylindrical gatebore and surrounding said plate outlet and engageable with saiddownstream flat surface to supplement the sealing engagement of saidplug seat surface aGainst said downstream flat surface when saidstructure occupies said second position; and a seal ring on said plugseat adjacent said upstream seat surface and engageable with the wall ofsaid cylindrical gate bore to prevent leakage of fluid from said inletpassages around said plug seat.
 11. In a valve as defined in claim 10;said gate having cylindrical surfaces conforming to and slidable alongthe cylindrical wall of said body bore between said segment seal plates.12. In a valve as defined in claim 10; and seal means on the exteriorsof said seal plates surrounding said inlet passages and outlet passagesand sealingly engaging the cylindrical wall of said body bore.